1. Field of the Invention
The present invention relates to a ship having a dome on its upper deck, and more particularly to a hull construction favorable for use in a liquefied gas carrying vessel in which a tank cover formed as a dome is additionally provided so as to cover a spherical tank projecting upwards from an upper deck.
2. Description of the Prior Art
Heretofore, liquefied gas carrying vessels employing a tank cover of hemispherical shape having a ribless structure have been known, and such tank cover structure is shown in FIGS. 8 to 12. In these figures, FIG. 8 is a transverse cross-sectional view of a hull, FIG. 9 is a partial longitudinal cross-sectional view of the hull, FIG. 10 is a partial top view of the hull, and FIG. 11 is a partial cross-sectional view taken along line a--a in FIG. 10 as viewed in the direction of the arrows.
Expansion joints 4 are disposed in the lengthwise direction as well as in the widthwise direction of a ship in a ribless tank cover 3 consisting of a hemispherical shell-like member that is provided on an upper deck 2 for the purpose of shielding a liquefied gas tank 1 from atmospheric air and sea water. The cross-sectional configuration of the expansion joint 4 is a closed rectangular shape as shown in FIG. 11. The expansion joints 4 are provided for the purpose of preventing buckling of the tank cover 3 due to high stress produced at the joint portion between the tank cover 3 and the upper deck 2, by absorbing deformation of the tank cover 3 and lowering stress produced in the tank cover 3 when the tank cover 3 is forcibly deformed due to deformation of the hull.
According to the knowledge of the prior art, it was considered to be natural that a structure largely influenced by deformation of a hull, such as the tank cover 3 (for instance, an upper structure of a car carrier or the like), be associated with a structure for absorbing deformation such as the above-mentioned expansion joint in order to avoid high stress caused by longitudinal bending deformation of the hull. However, in the case of tank cover 3 formed as a hemispherical shell-like member, the interference force exerted between the hull and the tank cover 3 is largest at the front and rear ends and at the left and right ends. Moreover, the expansion joints 4 are disposed just in the direction of connecting the front and rear ends and in the direction of connecting the left and right ends of the cover. Further, while the expansion joint 4 has good expansibility in its widthwise direction it is very rigid in its lengthwise direction, so that in the hatched portions shown in FIGS. 8 to 10 very large interference forces act between the tank cover 3 and the upper deck 2. With respect to practically built ships of the above-mentioned type cracks or break-down of welded portions area generated in such hatched portions.
It is to be noted that an annular upper deck portion, that is a portion of the upper deck 2 adjacent to the circumferential edge of a circular opening, does not include skelton members such as small ribs or girders and is formed as a flexible structure having brackets.
In addition, as shown in FIG. 12, a transverse bulkhead 16 in the heretofore known ship had a structure extended almost over the entire plane of the transverse cross-section of the hull. Hence, the rigidity of the hull is large, and the hull had such structure that the above-mentioned interference forces hardly could be absorbed by the side of the hull.
As described above, although expansion joints have been heretofore frequently used in a dome structure that is largely influenced by deformation of a hull such as a tank cover in a liquefied gas carrying vessel, there was the problem that difficulty with effectiveness of the expansion joints was liable to occur.
Hence, if a structure not provided with an expansion joint is employed, the above-mentioned difficulty would be eliminated, but, it would be necessary that the relative displacement between the hull and the dome, which was absorbed by the expansion joint in the prior art, be absorbed by a portion other than the expansion joint, or that the hull deformation per se be made small.